With the increasing down-scaling of integrated circuits and the increasingly demanding requirements to the speed of integrated circuits, transistors need to have higher drive currents with increasingly smaller dimensions. Fin Field-Effect Transistors (FinFETs), also known as multi-gate FETs, were thus developed. A typical FinFET includes a semiconductor fin above a substrate, which fin is used to form the channel region of the FinFET. The channel region includes sidewall portions and sometimes a top surface portion of the semiconductor fin. When the channel region includes the sidewalls portions but not the top surface portion, the respective FinFET is referred to as a dual-gate FinFET. When the channel regions include the sidewalls portions and the top surface portion, the respective FinFET is referred to as a tri-gate FinFET.
Some of the FinFETs are formed starting from silicon-on-insulator substrates. The respective FinFETs have reduced leakage currents. The manufacturing cost, however, is high. Some other FinFETs are formed starting from bulk silicon substrates. Accordingly, the resulting fins are connected to the bulk substrate through the silicon strips that are formed in Shallow Trench Isolation (STI) regions. Accordingly, leakage currents may flow through the semiconductor strips.
Conventionally, a heavy doping may be applied through an implantation, so that the dopants may be implanted into the silicon strips that are between STI regions. The heavy doping in the silicon strips help suppress the leakage currents. Such method, however, causes the respective dopant to be adversely introduced into the silicon fins that are overlying the silicon strips. Accordingly, the threshold voltages Vth of the respective FinFETs are reduced.